Fuel cells are receiving increasing attention as a viable energy-alternative. In general, fuel cells convert electrochemical energy into electrical energy in an environmentally clean and efficient manner. Fuel cells are contemplated as potential energy sources for everything from small electronics to cars and homes. In order to meet different energy requirements, there are a number of different types of fuel cells in existence today, each with varying chemistries, requirements, and uses.
As one example, Direct Methanol Fuel Cells (DMFCs) rely upon the oxidation of methanol on an electrocatalyst layer to form carbon dioxide. Water is consumed at the anode and produced at the cathode. Positive ions (H+) are transported across a proton exchange membrane to the cathode where they react with oxygen to produce water. Electrons can then be transported via an external circuit from anode to cathode providing power to external sources.
The kinetic and transport properties of the electrocatalyst and its integration into the fuel cell can have a profound effect on the efficiency and affordability of the fuel cell. The nature of the kinetic and transport properties of an electrocatalyst and its integration in the fuel cell is due in part to the hierarchical structure which combines distinct structural considerations across length-scales. On the scale of 1-10 nm, the key factors affecting activity include the crystallite size and identity of the exposed crystallite faces of the electrocatalyst. On the scale of 10-100 nm the topography of the electrocatalyst microstructure and/or electrocatalyst support are the significant elements. At this level, the transport channel for reactants and products is a direct consequence of the local structure. The local roughness at this level is associated with the surface area as well as the effective diffusivity of reactants and thus greatly influences the catalytic activity.
On the scale of 1-100 microns, the key features are of the electrocatalyst aggregates. The larger gas diffusion channels are a consequence of the agglomerated catalytic structure at this level. On the scale of 0.1-1 mm, the characteristic layer gradations of the fuel cell are apparent, namely the gas diffusion, catalytic and electrolytic membrane layers. Finally, at the scale of 1-10 mm, the fuel cell design and production irregularities become important.
Accordingly, finding novel electrocatalysts and methods for manufacturing them, can lead to significant advancement in fuel cells.
Currently, electrocatalysts can be separated into two basic types: supported electrocatalysts and unsupported electrocatalysts. Supported electrocatalysts comprise a highly dispersed metal composition or alloy such as platinum or platinum ruthenium that is supported by a carbonaceous framework. This high degree of dispersity of the metal increases the surface area of the metal, improving the efficiency of the catalytic reaction. Unsupported electrocatalyst remain attractive because they allow overcoming long-term durability issues associated with carbon support corrosion. n DMFCs, unsupported electrocatalysts have been commonly used, most usually unsupported Pt—Ru black. The amount of precious metal loading in a DMFC anode is in practice much greater than for a hydrogen fuel cell due to the greater complexity of the reaction. This cost can be sustained because the target market for DMFCs is small portable devices. An example of a commonly used unsupported electrocatalyst for DMFCs is Pt—Ru black.
One conventional method of electrocatalyst manufacture is by metallic impregnation or co-precipitation where an aqueous solution of the metallic precursor is contacted with the support substrate. (This method may be referred to herein as the “bulk” method of electrocatalysis synthesis.) After evaporation of the water, the metallic phase can be reduced under hydrogen flow for experimental use. The standard industrial method of producing fuel cell electrocatalysts is by adding a reducing agent, usually formaldehyde or formic acid, to an aqueous mixture of ionic metallic precursors which results in precipitation of the metallic electrocatalyst.
Another approach employs the synthesis of nanostructured materials and has centered on the replication of nanostructured silica templates followed by template removal. Many studies have focused on the synthesis of nanostructured carbon supports for electrocatalyst supports. Mesoporous carbon materials have been synthesized from several mesoporous silica materials. See e.g., R. Ryoo, S. H. J., M. Kruk, M. Jaroniec, Advanced Materials, 2001. 13: p. 677; and C. Yu, J. F., B. Tian, D. Zhao, G. Stucky, Advanced Materials, 2002, 14: p. 1742.
Some researchers believe that SBA-15 is the best template due to its interconnecting micropores between larger mesopores. Silica-templated mesoporous carbons with large surface areas have also been synthesized between the range of 10 nm to 10 μm depending on the choice of silica template. See e.g., Fuertes, A., Journal of Materials Chemistry, 2003. 13: p. 3085.
Various lengths and shapes of monometallic nanowires templated by various silica structures have been reported. See e.g., Bore, M., T. L. Ward, A. Fukuoka, Catalysis Letters, 2004. 98: p. 167-172; Egan, G. L., J. Yu, C. H. Kim, S. J. Lee, R. E. Schaak, T. E. Mallouk, Advanced Materials, 2001. 2: p. 1040; and H. J. Shin, C. H. K., R. Ryoo, Journal of Materials Chemistry, 2001. 11: p. 260.
Bimetallic structures templated by nanostructured silica have also been reported which include Pt—Rh and Pt—Pd nanowires. See e.g., A. Fukuoka, Y. S., S. Guan, S. Inagaki, N. Sugimoto, Y. Fukushima, K. Hirahara, S. Iijima, M. Ichikawa, Journal of the American Chemical Society, 2001. 123: p. 3373. Previous work on the synthesis of nanostructured Pt—Ru electrocatalysts includes the synthesis of a bimetallic Pt—Ru nanowire network templated by mesoporous silica, specifically SBA-15. See e.g., Choi, W. C. and S. I. Woo, Journal of Power Sources, 2003. 124(2): p. 420-425. The SBA-15 silica template is synthesized separately and is then immersed in a solution of Pt and Ru precursors. This infiltration procedure has to be repeated numerous times, usually about ten, before full pore infiltration can be accomplished See e.g., Choi et al, above. Following this approach, a significant increase in the electrocatalytic activity was found when compared to a commercial Pt—Ru black. Impregnation synthesis of a bimetallic network in mesoporous silica presents an interesting material design approach, although it is limited in application due to its complexity, parameter variability and time-consuming synthesis.
However, fuel cells employing the electrocatalysts formed by the methods described above are limited by material homogeneity and are laborious to synthesize. Accordingly, a novel electrocatalyst that can overcome or limit such inefficiencies is desirable. Specifically, it would be desirable to develop enhanced nanostructured materials that are homogeneous throughout the entire sample and which can be synthesized in a scaleable procedure.